1. Field of the Invention
The present invention relates to a mobile communication system. More particularly, the present invention relates to an apparatus and a method for allocating sub-carriers in a multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) mobile communication system.
2. Description of the Related Art
Since the development of a cellular wireless mobile telecommunication system in the United States at the end of the 1970's, voice communication service has been provided to users through an advanced mobile phone service (AMPS) system, which is a 1st generation (1G) analog type mobile communication system. Then, a 2G mobile communication system was developed in the middle of the 1990's. In addition, an International Mobile Telecommunication-2000 (IMT-2000), which is a 3G mobile communication system, was suggested at the end of the 1990's for the purpose of providing high-speed data services. Recently, IMT-2000 services have been partially provided to users.
Currently, the 3G mobile communication system is evolving into a 4G mobile communication system. Apart from previous mobile communication systems providing wireless communication services exclusively, the 4G mobile communication system may provide integrated wired/wireless communication services by effectively combining a wireless communication network with a wired communication network. To accomplish this, techniques for transmitting high-speed data services in the 4G mobile communication system are now being standardized.
When a signal is transmitted through a wireless channel in the above mobile communication systems, the transmitted signal is subject to multipath interference due to various obstacles existing between a transmitter and a receiver. Characteristics of the wireless channel having multipaths are determined by a maximum delay spread and a transmission period of a signal. If the transmission period of the signal is longer than the maximum delay spread, interference may not occur between continuous signals and a frequency characteristic of a channel is determined as frequency nonselective fading.
However, if a single carrier scheme is used when transmitting high-speed data having a short symbol interval, intersymbol interference may increase, causing signal distortion. Thus, the complexity of an equalizer at a receiving terminal may increase.
To solve the above problem of the single carrier scheme, an orthogonal frequency division multiplexing (OFDM) scheme has been suggested. According to the OFDM scheme, a multi-carrier is used to transmit data. The OFDM scheme is a type of a multi carrier modulation (MCM) scheme, in which serial symbol arrays are converted into parallel symbol arrays, which are modulated into a plurality of sub-carriers, that is, a plurality of sub-carrier channels which are orthogonal to each other.
A system employing the above MCM scheme has been adopted in a military HF radio for the first time at the end of the 1950's. In addition, the OFDM scheme for overlapping a plurality of orthogonal sub-carriers has been under development since the end of the 1970's. However, it is very difficult to realize orthogonal modulation between multi-carriers, so there is a limitation for directly applying the OFDM scheme to an actual system. In 1971, Weinstein et al. announced that modulation/demodulation using the OFDM scheme can be effectively performed by using Discrete Fourier Transform (DFT). From this point, techniques for the OFDM scheme have been rapidly developed. In addition, as a guard interval scheme and a cyclic prefix (CP) guard interval insertion scheme have been introduced, negative influences of the multipaths and delay spread upon a system can be further reduced.
Thus, recently, the OFDM scheme has been widely applied to digital transmission techniques such as digital audio broadcasting (DAB), wireless local area network (WLAN), and wireless asynchronous transfer mode (WATM). That is, the OFDM scheme, which is rarely used due to its complexity of hardware, can be adopted in the actual system as digital signal processing techniques, such as Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT), have been developed.
The OFDM scheme is similar to an Frequency Division Multiplexing (FDM) scheme. Among other things, the OFDM scheme transmits a plurality of sub-carriers while maintaining orthogonality therebetween, thereby obtaining an optimum transmission efficiency when transmitting high-speed data. In addition, since the OFDM scheme has superior frequency utilization and represents superior endurance against multipath fading, it may obtain optimum transmission efficiency when transmitting high-speed data.
In addition, since the OFDM scheme uses overlapping frequency spectrums, it can obtain superior frequency utilization. Furthermore, the OFDM scheme represents superior resistance against frequency selective fading and multipath fading, reduces an affect of Intersymbol interference (ISI) by using a guard interval, simplifies a structure of an equalizer, and reduces impulse-type noise. Thus, the OFDM scheme is positively utilized in a communication system.
Hereinafter, a structure of a communication system employing a conventional OFDM scheme will be described with reference to FIG. 1.
FIG. 1 is a view illustrating a transmitter of a conventional OFDM mobile communication system. The OFDM mobile communication system includes a transmitter 100 and a receiver 150.
The transmitter 100 includes an encoder 104, a symbol mapper 106, a serial to parallel converter 108, a pilot symbol inserter 110, an IFFT unit 112, a parallel to serial converter 114, a guard interval inserter 116, a digital to analog converter 118, and a radio frequency (RF) processor 120.
In the transmitter 100, user data 102 with user data bits and control data bits are output to the encoder 104. Upon receiving the user data 102, the encoder 104 codes user data 102 through a predetermined coding scheme and sends the data to the symbol mapper 106. Herein, the encoder 104 may code the user data 102 through a turbo coding scheme or a convolution coding scheme having a predetermined coding rate. The symbol mapper 106 modulates coded bits through a predetermined modulation scheme, thereby generating modulated symbols and sending the modulated symbols to the serial to parallel converter 108. Herein, the predetermined modulation scheme includes a binary phase shift keying (BPSK) scheme, a quadrature phase shift keying (QPSK) scheme, a quadrature amplitude modulation (16 QAM) scheme, or a quadrature amplitude modulation (64 QAM) scheme.
Upon receiving the serial modulated symbols from the symbol mapper 106, the serial to parallel converter 108 converts the serial modulated symbols into parallel modulated symbols and sends the parallel modulated symbols to the pilot symbol inserter 110. Thus, the pilot symbol inserter 110 inserts pilot symbols into the parallel modulated symbols and sends the parallel modulated symbols having the pilot symbols to the IFFT unit 112. Upon receiving signals from the pilot symbol inserter 110, the IFFT unit 112 performs N-point IFFT with respect to the signals and sends the signals to the parallel to serial converter 114.
Upon receiving the signals from the IFFT unit 112, the parallel to serial converter 114 converts the signals into serial signals and sends the serial signals to the guard interval inserter 116. The guard interval inserter 116, which has received the serial signals from the parallel to serial converter 114, inserts guard interval signals into the serial signals and sends the signals to the digital to analog converter 118. Insertion of the guard interval is necessary to remove interference between an OFDM symbol transmitted during a previous symbol transmission duration and an OFDM symbol to be transmitted in the present OFDM symbol transmission duration when OFDM signals are transmitted from an OFDM communication system. Such a guard interval has been suggested in such a manner that null data are inserted in the guard interval with a predetermined interval. However, when the null data are transmitted into the guard interval, if the receiver erroneously estimates a start point of the OFDM symbol, interference between sub-carriers may occur so that the probability of misjudgment for the received OFDM symbol may increase. Thus, a “cyclic prefix” scheme, in which predetermined last bits of an OFDM symbol in a time domain are copied and inserted into an effective OFDM symbol, or a “cyclic postfix” scheme, in which predetermined first bits of an OFDM symbol in a time domain are copied and inserted into an effective OFDM symbol, is used.
Upon receiving signals from the guard interval inserter 116, the digital to analog converter 118 converts the signal into an analog signal and sends the analog signal to the RF processor 120. The RF processor 131 includes a filter and a front end unit. The RF processor 131 transmits the signal output from the digital to analog converter 118 to air through a Tx antenna after RF-processing the signal.
Hereinafter, a structure of the receiver 150 will be described. The structure of the receiver 150 is reverse to the structure of the transmitter 100.
The receiver 150 includes an RF processor 152, an analog to digital converter 154, a guard interval remover 156, a serial to parallel converter 158, an IFFT unit 160, a pilot symbol extractor 162, a channel estimator 164, an equalizer 166, a parallel to serial converter 168, a symbol demapper 170, a decoder 172 and a data receiver 174.
First, noise is added to the signal transmitted from the transmitter 100 while the signal is being passed through a multipath channel. Then, the signal is transmitted to the receiver 150 through a reception antenna. The signal received through the reception antenna is input into the RF processor 152. The RF processor 152 down-converts the signal received through the reception signal such that the signal has an intermediate frequency band and sends the signal to the analog to digital converter 154. The analog to digital converter 154 converts the analog signal of the RF processor 152 into a digital signal and sends the digital to the guard interval remover 156.
Upon receiving the digital signal from the analog to digital converter 154, the guard interval remover 156 removes the guard interval signals and sends serial signals to the serial to parallel converter 158. The serial to parallel converter 158, which received the serial signals from the guard interval remover 156, converts the serial signals into parallel signals and sends the parallel signals to the IFFT unit 160. The IFFT unit 160 performs an N-point IFFT with respect to the parallel signals output from the guard interval remover 156 and sends the signals to the equalizer 166 and the pilot symbol extractor 162. Upon receiving the signals from the FFT unit 160, the equalizer 166 performs channel equalization with respect to the signals and sends the signals to the parallel to serial converter 168. The parallel to serial converter 168 converts the parallel signals into serial signals and sends the serial signals to the symbol demapper 170.
In the meantime, the signal output from the IFFT unit 160 is input into the pilot symbol extractor 162 so that the pilot symbol extractor 162 detects pilot symbols from the signals of the IFFT unit 160. The pilot symbols detected by the pilot symbol extractor 162 are sent to the channel estimator 164. Thus, the channel estimator 164 performs channel estimation by using the pilot symbols output from the pilot symbol extractor 162 and sends the channel estimation result to the equalizer 166. In addition, the receiver 150 generates channel quality information (CQI) corresponding to the channel estimation result and sends the CQI to the transmitter 100 through a CQI transmitter (not shown).
The symbol demapper 170 demodulates the signals output from the parallel to serial converter 168 through a predetermined demodulation scheme and sends the decoded signals to the decoder 172. Upon receiving the demodulated signal from the symbol demapper 170, the decoder 172 decodes the demodulated signals through a predetermined decoding scheme, and then, outputs the demodulated signals. The demodulation and decoding schemes employed in the receiver 150 correspond to the modulation and coding schemes employed in the transmitter 100.
Recently, the OFDM scheme has been actively studied as a representative communication scheme for a 4G mobile communication system and a next generation communication system. As described above, the OFDM uses a plurality of sub-carriers having orthogonality in order to improve frequency utilization, employs Inverse Fast Fourier Transform and Fast Fourier Transform (FFT) so as to easily process high-speed data, and utilizes a “cyclic prefix” in order to improve endurance against multi-path fading. In addition, the OFDM scheme is applicable for a multiple-input and multiple-output (MIMO) system.
The OFDM scheme used for a single user is different from the OFDM scheme used for a plurality of users. The OFDM scheme used for a plurality of users must allocate sub-carriers such that the sub-carriers do not overlap each other while taking the transmission rate and transmission power per each user into consideration. Thus, various sub-carrier allocation methods have been suggested for the OFDM schemes. One of the methods is to minimize total transmission power while using a bit rate for each user as a constraint by using an algorithm based on a Lagrange optimization method. However, although the Lagrange optimization method can attain an optimal solution, it is complex and has a low convergence speed.
In order to obtain faster speeds, a two-step sub-carrier allocation scheme has been suggested, in which the number of sub-carriers and transmission power to be allocated to each user are determined and the sub-carriers are allocated to each user such that a maximum data transmission rate can be obtained. At this time, a Hungarian algorithm is used for the sub-carrier allocation, resulting in complexity even if the number of users is small. Besides, a water-filling algorithm is used for multi-users.
There is a limitation to obtaining an optimal antenna diversity gain from a single antenna structure as shown in FIG. 1. Thus, a diversity scheme using multiple antennas has been proposed. A transmit diversity scheme using multiple antennas includes an open-loop scheme and a closed-loop scheme. The open-loop scheme has no feedback information and includes a space-time scheme including a space-time block code (STBC), a space-time trellis code (STTC) and a layered space-time code. The closed-loop scheme has feedback information and includes a transmit antenna array scheme in which each antenna transmits sub-carriers while adding a weight to sub-carriers by calculating the weight based on channel information of the signal transmitted through the antenna.
An antenna selective transmit diversity scheme can be utilized in relation to the open/closed loop schemes. In a case of the open-loop scheme, the same number of sub-carriers is allocated to each antenna. For instance, if two antennas are provided, odd sub-carriers are allocated to the first antenna and even sub-carriers are allocated to the second antenna. In a case of the closed-loop, the sub-carrier is allocated to an antenna having a superior channel characteristic by using channel information of the antennas, so the same number or a different number of sub-carriers is allocated to each channel.
Various algorithms have been suggested for allocating sub-carriers in the OFDM scheme. However, such algorithms are applicable only for a single antenna structure and target two-dimensional (time-space) resource allocation. Studies for a three-dimensional (time-frequency-space) resource allocation algorithm under a real multiple antenna environment are rarely performed. In addition, when allocating sub-carriers to the antenna through the OFDM scheme, the single antenna structure is necessary to use a plurality of sub-carriers for one antenna so a peak to average power ratio (PARR) may increase.
If the PARR increases, the complexity of an analog to digital converter (ADC) and a digital to analog converter (DAC) also increases and an efficiency of a radio frequency power amplifier may be reduced. A coding scheme and a clipping scheme are proposed in order to reduce the PARR. However, not only is it difficult to find a code capable of reducing the PARR in relation to a plurality of sub-carriers, but also interference between an in-band frequency and an out-of-band frequency may increase. In addition, multiple antennas can be used to reduce the PARR. If the same number of sub-carriers is allocated to each antenna, the PARR can be reduced in a ratio of 10 log N/n with respect to N sub-carriers, herein N is a number of sub-carriers and n is a number of antennas. However, it is necessary to effectively adjust the number of sub-carriers when the sub-carriers are allocated to each antenna.
In the meantime, transmission power must be considered in the OFDM scheme in addition to the PARR. Under the multi-user environment, power of the sub-carrier allocated to each user must be properly adjusted in order to reduce interference between users. Since the open-loop scheme of the transmit diversity method does not receive feedback information from a receiving terminal, an overhead may not occur. However, it is necessary to transmit the sub-carrier to an antenna even in an inferior channel state of the antenna, so performance degradation may occur. The closed-loop scheme allocates the sub-carrier to each antenna based on a channel state of the antenna so the closed-loop scheme represents superior performance. However, the closed-loop scheme may induce overhead due to feedback information thereof, so it is necessary to reduce the overhead. In addition, the closed-loop scheme optimizes transmission power by using a variety of channel estimation information, so a complex algorithm is required. In particular, the Lagrange optimization algorithm represents complexity which is almost impossible to realize in an actual system. In addition, various simplified sub-optimal algorithms have been suggested, but unnecessarily require overlapping loops and a plurality of sorting operations.